To date, lyophilizer systems have included the use of a cold trap condenser. The condenser is designed to capture the full volume of ice sublimating from frozen product during lyophilization. After product is frozen on the freeze dryer shelves a vacuum is applied to the product chamber lowering the vapor pressure of the ice present. This action results in the initiation of sublimation. The low pressure created by pulling a vacuum allows water molecules to diffuse directly from the solid state "ice" to the gas state "water vapor." Since sublimation of ice to water vapor takes energy, the shelves must be heated to continue the process. Water molecules will continue to sublimate unless an equilibrium is reached between the water molecules present as vapor in the chamber and those sublimating from the ice. To prevent an equilibrium condition from affecting the sublimation rate, water vapor must be removed from the processing chamber. By removing the water vapor, a diffusion gradient will be maintained between the product and the environment within the chamber. Other methods presently used for capturing water vapor during freeze drying include the use of brine solutions and desiccants. These methods both work by indirect water vapor removal from the processing environment.
In present lyophilizing systems, once sublimation is initiated, water vapor is recaptured by a cold trap condenser prior to it reaching the vacuum pump. The condenser acts as a "water molecule pump" for the condensable gas "water vapor." The condenser captures condensable water vapor released during sublimation by removing heat from the vapor. Removing the heat causes the vapor to recrystallize onto the condenser plates as ice. This process provides the means by which a diffusion gradient is maintained between the product shelves and the condenser so that sublimation will continue. The condenser is normally operated at a temperature significantly lower than that of the product shelves. Heat energy must be supplied to the product shelves at a rate that will allow ice in the product to sublimate while maintaining the product temperature below its freezing point.